The overall goal of this work is to continue to examine the differential response to anterior cruciate ligament (ACL) rupture. Although ACL reconstructive surgery has had reasonable success, it is not clear why these patients are more likely than uninjured subjects to develop osteoarthritis (OA) in later years. Based on the results of our first period of funding, we believe this is due to altered muscle morphology and activation. The changes in morphology, which can be measured using imaging techniques, lead to changes in muscle paths, muscle moment arms, and muscle volumes, which reduce force generating capacities in muscles. Such changes occur in selective muscles following reconstructive surgery. The changes in neural activation can be measured using EMGs, and result in different distributions of forces within the knee, which may lead to OA. In this proposal, we plan to use MRI and biomechanical modeling to examine ACL injured knees before and after reconstructive surgery. The aim of this study is to characterize changes in (1) muscle/tendon morphology, (2) extensor and flexor strength, and (3) joint stabilization function, after post-rupture surgical reconstruction of ACL using bone-patellar tendon-bone autograft (BPTB), and quadruple-bundled semitendinosus gracilis (QSTG) autografts. We hypothesize that the morphology, activation and forces in the muscles about the knees will be different in the ACL reconstructed knees compared to unimpaired knees. Recovery from ACL injury is a major health problem, affecting over 80,000 Americans a year. Better understanding of the recovery from this surgery may reduce future incidents of OA in these patients.